A Cu-Zeolite SCR system was experimentally compared with a Fe-Zeolite one in terms of NOx conversion efficiency over a wide range of exhaust gas temperatures in a heavy-duty diesel engine. The test results show that the Cu-Zeolite SCR can reduce NOx more effectively due to increased NH₃ adsorption at lower temperatures than the Fe-Zeolite type. In addition, N₂O tends to be formed in a DOC located downstream of the SCR system depending on slipping NH₃ from it as well as on the lower temperatures. A simplified catalytic reaction model developed in this study can explain these SCR and N₂O formation reactions.